Such a cogeneration system is known from, for example, Japanese Patent Application Laid-open No. 4-342807 in which, after the thermal energy of exhaust gas discharged from a prime mover is recovered by a waste heat boiler, air that is to be used in an air conditioner is heated by heat exchange with the exhaust gas in an air-heating heat exchanger, thus recovering the waste thermal energy effectively.
In the air-heating heat exchanger, which carries out heat exchange with the exhaust gas on the downstream side of the waste heat boiler, part of the water contained in the exhaust gas is condensed by being cooled by heat exchange with the air that is to be fed into the air conditioner, and is discharged as drain water, and although the temperature of the drain water is expected to be comparatively high, there is no conventional system in which the thermal energy is recovered effectively from such drain water.